1. Field of the Invention
This invention relates to the conversion of heavy, sulfur-containing, petroleum feedstocks and more particularly to processes for coking sulfur-containing residual petroleum feedstocks with added inorganic material containing an alkaline earth metal oxide or a substance forming alkaline earth metal oxides under coking conditions. This invention also relates to a clean-burning, fixed-sulfur, solid fuel product and method for its production.
2. Description of the Prior Art
Coking is an increasingly important processing area in petroleum refining. As high quality crudes become scarcer and more expensive, refineries must process increasing quantities of lower quality crudes which contain or, upon processing, form large amounts of high-boiling materials that are typically treated in coking units. Thus, the quality and quantity of products produced by coking processes can have a large impact on overall refinery yields because the relative amount of feedstock to be coked generally increases as the quality of crude oil material decreases.
Principle heavy petroleum coking feedstocks are high-boiling virgin or cracked petroleum residua such as virgin reduced crude, bottoms from vacuum distillation (vacuum reduced crude), thermal tar and other residue and blends thereof. Coking enables efficient conversion of these less desirable petroleum fractions to more desirable distillate products and a byproduct coke.
A variety of coking methods are known in the art including delayed, fluid, and moving bed coking processes.
Delayed coking is a process wherein the feedstock is preheated to a coking temperature, generally between 800.degree. F. to about 1100.degree. F. and more usually between about 850.degree. F. to 950.degree. F. The preheated feedstock is then fed to the bottom of a delayed coker drum. The coking feed is allowed to soak in its own heat in the delayed coker at a low pressure, generally from about one atmosphere to about 10 atmospheres absolute, preferably from about three atmospheres to about seven atmospheres absolute. The cracked vapors are continuously removed overhead so as to recover the distillate fuels while coke is allowed to build up in the drum to successive higher levels. When the drum is filled with coke, the preheated feed is diverted to a succeeding drum and the former drum is steamed out and cooled. The coke is then removed from the cooled drum.
Fluid coking is a process wherein feedstock is sprayed into a bed of hot fluidized coke particles in a reactor. The feedstock is cracked into lighter vapor-phase products and into coke, the coke being deposited on the particles of the fluidized bed. The particles of coke are circulated from the reactor to a burner wherein they are partially combusted with an oxygen-containing gas in a moving, fluid, or transfer line combustion zone and thereby raised in temperature, some of the heated coke particles being returned to the reactor for further use, the remainder of the coke being withdrawn as a byproduct. In a typical fluid coking unit the feedstock is converted to about 70% of normally liquid products and about 25% of coke, and 7-8% of the latter (based on charge) is consumed in the burner to provide heat for the process.
Moving bed coking is a process wherein the feedstock is uniformly distributed to the top of a mass of heated granular petroleum coke particles maintained in a reactor through which the particles downwardly pass by gravity. The liquid hydrocarbon charge is converted by the heat of the particles to produce lower-boiling vapors and a dry coke coating on the particles. The coated coke particles are withdrawn from the bottom of the reactor and either recovered as a coke byproduct or passed to a burner similar to that employed in fluid coking processes to raise the coke particle temperature for return to the coking reactor.
Sulfur compounds present in crude petroleum include thiols (mercaptans) and open-chain and cyclic sulfides. Other compounds such as thiophenes and aromatic thiols may be present in cracked petroleum products but their presence in naturally-occuring petroleum is doubtful. As the sulfur content of the crude increases, there is a tendency toward sulfur distributions in distillation products wherein as much as 90% or more of the sulfur content of the crude is present in distillation residues. Because distillation residues are typically treated in coking units, the problems presented by sulfur contaminants in the refining of petroleum have special importance in coking processes.
When coking feedstocks containing sulfur and other materials such as vanadium and sodium which are generally regarded as deleterious contaminants, the sulfur contaminants tend to be roughly equally distributed between the solid, normally liquid, and normally gaseous products and the other contaminants tend to be concentrated in the solid coke product. As a result, the uses to which the coke may be put are restricted. For example, when coke containing sulfur is burned, the sulfur is liberated as sulfur oxides which are noxious and corrosive, presenting a serious atmospheric pollution problem. The difficulties associated with sulfur contamination are accentuated by the present trend toward the use of higher sulfur content crude oils in many refineries.
Several alternatives have been suggested for overcoming this problem. More particularly, a wide variety of processes have been proposed for producing desulfurized petroleum coke. For example, U.S. Pat. Nos. 2,768,939 and 3,130,333 suggest desulfurizing coke by hydrogenation. U.S. Pat. No. 2,824,047 suggests desulfurization of char or coke with hydrogen in the presence of an "acceptor" compound for hydrogen sulfide capable of maintaining a low ratio of hydrogen sulfide to hydrogen in the operation. Various substances containing lime were proposed for the acceptor, including calcined dolomite. A similar method is disclosed in U.S. Pat. No. 3,481,834 which suggests a process for converting sulfurous petroleum residual oil into low sulfur products by coking the liquid hydrocarbons in a first zone containing a fluidized bed of coke pellets and maintaining a separate but contiguous zone comprising a fluidized bed of a solid containing a substance "avid to receive sulfur from hydrogen sulfide", wherein the coked solids from the first zone and "sulfur-avid" solids comingle. Spent solid "sulfur-avid" substances, which may be oxides of calcium, manganese, iron, lead, or copper are removed from the second zone and regenerated. For example, calcium sulfide is converted to calcium carbonate and hydrogen sulfide by treatment with steam and carbon dioxide at elevated pressure and at a temperature below 1300.degree. F. in a "reducing" atmosphere to avoid explusion of sulfide dioxide and to prevent oxidation of calcium sulfide to form calcium sulfate.
Methods have also been proposed for producing desulfurized petroleum coke which comprise adding various materials to the coking zone itself.
U.S. Pat. No. 2,921,017 suggests mixing powdered sodium carbonate with a coker feedstock, coking resulting mixture in a "calciner coker" at about 1400.degree. F., washing the resulting coke product, and recovering a desulfurized coke product. Alternatively, the '017 patent suggests initially coking the sodium carbonate/coker feedstock mixture at a temperature of about 900.degree. F., raising the temperature of the resulting sodium carbonate/coke mixture to 1400.degree. F. in a separate vessel to complete the coking and desulfurization, and washing the resulting solid.
U.S. Pat. No. 2,968,611 suggests adding desulfirizing agents such as aluminum hydrate, boron oxide, iron oxide, clays or bauxite to a heavy hydrocarbon charge to a moving bed coker.
U.S. Pat. No. 3,723,291 suggests a method of producing coke of reduced sulfur content by adding an alkali metal carbonate to coker feedstock prior to coking and then, after coking, treating the coke product with hydrogen at elevated temperatures (1000.degree. to 2000.degree. F.), releasing sulfur from the coke as hydrogen sulfide.
U.S. Pat. No. 3,873,427 suggests adding a desulfurizing agent containing iron or iron oxide and a chloride of magnesium, calcium or iron to a coker feedstock, coking the resulting mixture in a delayed or fluid coker, desulfurizing the resulting coke in conventional calcining equipment at temperatures of at least 2100.degree. F. under reducing conditions, and recovering a coke product containing no more than 0.85% sulfur.
U.S. Pat. No. 3,907,662 discloses a method for producing desulfurized light oil and fuel gas from heavy oil which comprises coking and heavy oil in a fluidized bed of particles comprising an alkali metal carbonate compound maintained at a temperature of 470.degree.-550.degree. C.; recovering volatile products from the first low-temperature fluidized coking zone; passing the carbonaceous particles formed in the first zone to a second fluid bed coking zone maintained at a temperature of 600.degree.-800.degree. C. and recovering additional volatile products and forming coke solids containing alkali metal sulfides; regenerating the alkali metal sulfide solids and recovering fixed sulfur as hydrogen sulfide; heating and gasifying the regenerated alkali metal carbonate/coke particles to form a heated fuel gas; and returning the particles containing alkali metal carbonate, from which most of the coke has been removed, to the second, high-temperature fluid bed coking zone. At column 9, line 10-13, the patent teaches that other particles having desulfurization effect can be used in addition to the alkali metal carbonate compound and suggests dolomite as an example of such a compound.
U.S. Pat. No. 3,915,844 discloses an improved fluid coking method for the treatment of heavy hydrocarbon feedstocks wherein particles consisting of an alkali metal compound heated to a temperature of 100.degree.-500.degree. C. higher than the temperature of the coking zone are added to the coking zone to seed formation and growth of coke. The coke product is gasified and particles comprising alkali metal compound are returned to the coking zone. In one embodiment of this process, the alkali metal compound is supported on an alkaline earth metal compound, i.e., calcium oxide, calcium carbonate, magnesium oxide, magnesium carbonate and dolomite.
U.S. Pat. No. 3,707,462 suggests a method of converting a sulfur-containing, heavy petroleum feedstock which method comprises coking the feedstock in a first zone containing a fluidized bed of calcium oxide or a precursor thereof at a temperature of between 500.degree.-700.degree. C., recovering vapors comprising normally liquid and gaseous products of reduced sulfur content and carbonaceous material of increased sulfur content which deposits on the calcium oxide particles from the first zone, partially removing the carbonaceous deposits formed on the calcium oxide particles in a second zone by combustion of a fluidized bed of the solids at a temperature 800.degree.-1000.degree. C., transferring some of the partially combusted solids from the second zone to the first zone, transferring the remainder of the partially combusted solids from the second zone to a third zone wherein the particles are fluidized at a temperature 1000.degree.-1100.degree. C. in an oxygen containing gas to convert at least some of the calcium sulfide present to calcium oxide with the release of sulfur dioxide, and transferring the desulfurized oxidized particles from the third zone back to the first zone. The patent teaches that the temperature and oxidizing potential in the second zone should be maintained such that the carbonaceous deposit is substantially removed from the particles therein without causing sulfur to be lost from the particles.
Although it is not concerned with sulfur removal, U.S. Pat. No. 2,953,518 discloses an improved fluid coking operation which comprises employing a seed material consisting essentially of calcium oxide in amounts equal to about 45-75% of the fluidized bed. This process is said to minimize agglomerating tendencies in the coking zone and to produce a solid byproduct suitable for the manufacture of calcium carbide. A portion of the solid, coke-lime byproduct is burned and returned to the fluid coking zone to provide the necessary heat of reaction.